“Dadirri” - 71a power amp.

 

Every now and then you build an amp which stands out from all the other builds you have done in the past. Of the 71a amps I have built there is one for me and now two for others. Though there is a list of standard parts used in this build for some reason it sounds way better than it should. I did use new driver tubes - Tung-Sol 6SN7 - but the other tubes, 71a Cunningham and RCA 80 rectifier tube are vintage and I have used them before. I guess just accept it, listen and enjoy and don’t over think it. I know the new owner will be very pleased. He is using his own tubes. 

The timber chassis I am now buying from Aliexpress and though pricey they are solid and very good. In this build I left the top plate natural low sheen Al. It would be a shame to paint this one. An oversize Hammond power tranni and Asian OPTs round out the iron. Carbon film 1W Rs, wire wounds and a few metal film Rs were used. Interstage caps are Russian PIO in glass and metal envelope. I wired 4 and 8ohm speaker connections in this build. 

The 5V DC heater voltage for the 71a tubes is provided by a 3A bridge and two 4700uf caps and taps off from the 6.3V AC filament voltage for the 6SN7s. All wiring is with solid core Cat5 and no shielded wire is used. Noise is less than 1mV. HT is rectified by the 80 tube and a double Pi filter cleans up the 220V DC reaching the 71a tubes. There is 247uf of storage and smoothing caps. The HT feeding the driver tubes is split in two and 2X 10uf 450V caps hold a fast and individual power storage. The stereo imaging of this amp is extremely good and it may be due to this HT split to the drivers. 

I’m extremely pleased with the overall appearance of this amp. Matching the good looks is the most beguiling sound I have heard from a tube amp which only supplies about 300mW of power but in my system plays quite loud with my 93db efficient speakers. This one should have been a keeper.

50W Class A Holton based power amp

 

For a long time friend, who already has two of my power amps, one SS and one tube, I was requested to build this Class A power amp. Holton Precision of Tasmania had developed some new Class A 50W modules. That’s 50W pure Class A before sliding across to B. Amp should produce 90W in Class B. 

The PS consisted of a 300W tranni and 132,000uf of Mundorf Mlytic audio grade caps. Dual 35A rectifier bridges round out the PS. The caps were heavily snubbed with high voltage polies. With a result the amp has near zero AC noise. But better than that, the amp produces beguiling music like I’d not heard before. Its ability to dig out inner detail and bring it to the front was uncanny. The new owner liked it also. 

The amp runs extremely hot and to help control the heat two silent running fans were installed. The fans sat on rubber grommets for isolation and were run on low voltages. With a result they could not be heard. Though this build produced many challenges, over months of construction, it was worth it.